This invention concerns a spark-discharge apparatus. More particularly, this invention concerns an apparatus for passing a spark discharge between a conductive workpiece to be tested and an electrode.
It is known to test the composition of an alloy workpiece by passing a unipolar or single-polarity pulse between a sample of this workpiece which is in contact with an electrode and a counter electrode spaced from the sample. The spark produces light in both the visible and ultraviolet spectrum which can be divided into spectral lines each of which represents a component of the workpiece and the intensity of each line representing the quantity of that component in the workpiece. Thus a spectrometer working with such a spark-discharge arrangement can perform an exact quantitative and qualitative analysis of an alloy workpiece.
The production of such a spark requires very high voltage, so that the sample must normally be placed in a housing provided with safety-interlocks that prevent the device from functioning unless the housing is closed and, therefore, all operating personnel are out of danger. Furthermore, if the short wavelength of the ultraviolet part of the spectrum is of interest the test requires to be carried out in an air-free environment. Thus it is necessary to provide a large vacuum bell and associated pumping equipment. For this reason such a device is normally very large and cannot be carried to the site.
It is common practice with such arrangements to cut a small piece off all of the workpieces to be tested and carry them to the lab where the device is used. Not only does such a testing method take considerable manpower, but it usually does substantially more damage to the workpieces than is necessary for the test, as only a very small quantity of the workpiece need be vaporized by a spark to obtain the necessary information. For this reason the above-described testing method is employed as little as possible.
Some of these problems are overcome by not evacuating the apparatus, as the very short waves produced by, for example, carbon and sulfur, frequently are not of interest. For identification purposes, it is the longer ultraviolet rays produced by silicon, chromium, manganese, molybdenum, and the like that are of interest.
Another arrangement has been developed which generates a very hot spark by using a bi-polar discharge across the gap. This allows the workpiece to be analyzed in situ, however, the danger of shock to the operating personnel remains great. Furthermore, the use of a bi-polar pulse causes the counter electrode as well as the workpiece to vaporize. Some of the workpiece inevitably settles on the counter electrode so that when the next test is made it is necessary either to provide a new electrode, refinish the electrode, or make several tests to burn off the material from the earlier test. Absent these precautions each test will be somewhat contaminated by the previous tests.